A parallel signal processing (PSP) method has been proposed for ultrasonic diagnostic apparatuses to increase a frame rate. In the PSP method, an ultrasonic diagnostic apparatus transmits an ultrasonic transmission beam in a transmission direction. The transmission beam is reflected by a subject. The ultrasonic diagnostic apparatus then receives the reflected ultrasonic waves and generates echo signals corresponding to the received ultrasonic waves. The ultrasonic diagnostic apparatus generates a plurality of reception beam data sets respectively corresponding to a plurality of reception beams based on the generated echo signals. In this manner, in PSP, the ultrasonic diagnostic apparatus simultaneously receives a plurality of reception beams with respect to one transmission of a transmission beam. This increases the amount of data per unit time.
FIG. 11 is a view showing the positions of reception beams in a standard scanning scheme for PSP. The following description will exemplify the simultaneous reception of eight beams. Note that the simultaneous reception of eight beams is a scanning scheme for simultaneously receiving eight reception beams with respect to the transmission of one transmission beam. The abscissa represents the transmission direction of transmission/reception beams; and the ordinate, the depth direction of transmission/reception beams. In this case, eight reception beams to be parallelly and simultaneously received at once will be collectively referred to as a PSP beam group. A plurality of reception beam data sets associated with a PSP beam group will be collectively referred to as a PSP beam data set group. In addition, a series of processing operations from the transmission of one transmission beam to the reception of reception beams will be referred to as one transmission/reception.
Referring to FIG. 11, eight reception beams RB to be parallelly and simultaneously received in the first transmission/reception are represented by the number “1”, and the reception beams in a PSP beam group PR are represented by “a”, “b”, “c”, “d”, “e”, “f”, “g”, and “h” from the left in the scanning order. As indicated by (1) in FIG. 11, in the first transmission/reception, a transmission beam TB is transmitted. The central axis of the transmission beam TB in the first transmission/reception is located in the middle between “1d” and “1e”. The eight reception beams RB indicated by “1a”, “1b”, “1c”, “1d”, “1e”, “1f”, “1g”, and “1h” are parallelly and simultaneously received. The eight reception beams RB are arranged at equal intervals in the scanning direction. More specifically, the interval between the center positions of the adjacent reception beams RB (the interval between the central axes) is set to one reception beam. In this case, the position of the central axis of the reception beam RB on a probe will be referred to as a reception position. In other words, a reception position is defined as the center of the reception opening used for the reception of the reception beams RB. As indicated by (2) in FIG. 11, the central axis of a second transmission beam TB2 is located in the middle between “2d” and “2e”, which is shifted from the central axis of a first transmission TB1 by eight reception beams in the scanning direction. In the second transmission/reception, eight reception beams represented by “2a”, “2b”, “2c”, “2d”, “2e”, “2f”, “2g”, and “2h” are parallelly and simultaneously received. Note that the scanning scheme in which the interval between the central axes of two adjacent PSP beam groups corresponds to eight reception beams is called “8-beam shifting”. In this case, two adjacent PSP beam groups PR1 and PR2 do not spatially overlap, and hence beam synthesis (to be described later) is not performed.
In PSP, however, since the position of the central axis of a transmission beam differs from that of each reception beam, the transmission/reception sensitivity deteriorates, and it is impossible to obtain uniform transmission/reception sensitivity.
FIGS. 12 and 13 are views for explaining reception sensitivity nonuniformity in the scanning scheme in FIG. 11. As indicated by (2) in FIG. 12 or (2) in FIG. 13, the sound pressure in the transmission sound field at an end portion of the transmission beam TB in the scanning direction is lower than that at the middle portion. For this reason, if the number of reception beams to be parallelly and simultaneously received is set to three or more, transmission/reception sound fields become nonuniform in magnitude (i.e., reception sensitivity) in the scanning direction. As indicated by (2) in FIG. 12 or (2) in FIG. 13, therefore, the reception beams at the middle portion and end portions of the PSP beam group PR become uneven in reception sensitivity. Unevenness in reception sensitivity will lead to a deterioration in the image quality of an ultrasonic image.
In PSP, the PSP beam group obtained by preceding transmission/reception differs in phase from that obtained by succeeding transmission/reception. FIG. 14 is a view for explaining phase differences between PSP beam groups in the scanning scheme in FIG. 11. As shown in FIG. 14, the phases of reception beams switch every time PSP beam groups switch, that is, for every eight reception beams. In this case, the phases of beams are synonymous with the order (numbers) of beams generated in transmission/reception. For example, a plurality of reception beams in the first transmission/reception all belong to the same phase. The PSP beam group in the first transmission/reception differs in phase from that in the second transmission/reception.
FIG. 15 is a view for explaining the scanning scheme in FIG. 11 (simultaneous reception of eight beams & no beam synthesis & eight-beam shifting). As shown in FIG. 15, in the first transmission/reception, eight reception beams (“1a”, “1b”, “1c”, “1d”, “1e”, “1f”, “1g”, and “1h”) are received. These eight reception beams constitute a PSP beam group PB1. Subsequently, likewise, PSP beam groups PB2, PB3, and PB4 are respectively received in the second, third, and fourth transmissions/receptions. The respective reception beams are sequentially output (in the reception order) without beam synthesis. Assume that the sensitivity of reception beams at positions “a” and “h” is 7, the sensitivity of reception beams at positions “b” and “g” is 8, the sensitivity of reception beams at positions “c” and “f” is 9, and the sensitivity of reception beams at positions “d” and “e” is 10. As shown in FIG. 15, transmission/reception sensitivity nonuniformity (reception sensitivity unevenness) occurs between the reception beams of each PSP beam group PB in accordance with the distances from the central axis of each PSP beam group. In addition, since adjacent PSP beam groups do not spatially overlap, the phases of output beams (reception beams) switch for each PSP beam group. Such transmission/reception sensitivity nonuniformity and phase differences will produce artifacts in ultrasonic images in a density stripe pattern, resulting in a deterioration in image quality. This deterioration in image quality worsens with an increase in the number of PSP beams.
In order to solve this problem, there have been proposed a transmission delay addition technique and a transmission wavefront synthesizing technique. FIG. 16 is a view showing the positions of reception beams in another scanning scheme for PSP (parallel signal processing of eight beams & eight-beam synthesis & one-beam shifting). FIG. 17 is a view for explaining the scanning scheme in FIG. 16. As shown in FIGS. 16 and 17, the central axis of a first transmission beam TB1 is located in the middle between “1d” and “1e”. The central axis of a second transmission beam TB2 is located in the middle between “2d” and “2e” shifted from the central axis of the first transmission beam TB1 by one reception beam in the scanning direction. At this time, for example, reception beams RB respectively indicated by “1h” and “2g” are located at spatially the same reception position. At the end of the eighth transmission/reception, eight reception beams RB belonging to a different PSP beam group (in other words, at different reception times) are obtained. For example, at a reception position PZ, eight reception beams indicated by “1h”, “2g”, “3f”, “4e”, “5d”, “6c”, “7b”, and “8a” are obtained. Synthesizing the eight reception beams at spatially the same position will generate a synthetic beam. For example, synthesizing the reception beams indicated by “1h”, “2g”, “3f”, “4e”, “5d”, “6c”, “7b”, and “8a” will generate a synthetic beam O1. Repeating transmission/reception while shifting the central axis of the transmission beam TB by one reception beam at a time in this manner will receive eight reception beams at spatially the same position and generate a synthetic beam based on the eight reception beams. This makes the reception sensitivity of an output beam (synthetic beam) after synthesis uniform in the scanning direction. In addition, since reception beams are synthesized as PSP beam groups spatially overlap, the phases of synthetic beams switch for each synthetic beam. It is therefore possible to reduce artifacts in a stripe pattern due to phase differences on a PSP beam group basis.
However, since one synthetic beam is generated by synthesizing a plurality of reception beams, the larger the number of reception beams to be synthesized, the lower the frame rate. In addition, as the number of reception beams to be synthesized increases, the phase differences between the reception beams to be synthesized increase. This makes it difficult to apply this technique to fast moving regions such as the heart.
FIG. 18 is a view for explaining a still another scanning scheme for PSP (parallel signal processing of eight beams & four-beam synthesis & two-beam shifting). The number of reception beams to be synthesized in the scanning scheme in FIG. 18 is decreased from eight to four to increase the frame rate. In this case, it is possible to simultaneously increase the frame rate and reduce artifacts in a stripe pattern due to phase differences on a PSP beam group basis. However, since two reception beams are shifted at a time in the scanning direction, the phases of synthetic beams switch for every two synthetic beams instead of one synthetic beam. This causes image shifts in an ultrasonic image, resulting in an unnatural image.
It is an object of the embodiment to provide an ultrasonic diagnostic apparatus and ultrasonic transmission/reception method which can improve image quality in PSP.